Correct value for --format in README

[afinetooth]

:zap: Summary

Corrects --format value for CloverXML / PHPUnit. Should be clover instead of phpunit.

:ballot_box_with_check: Checklist

• [x] Correct --format value for CloverXML / PHPUnit

[coveralls-official[bot]]

Pull Request Test Coverage Report for Build 6870949630

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 93.719%

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Totals
Change from base Build 6721884478:	0.0%
Covered Lines:	940
Relevant Lines:	1003

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💛 - Coveralls

[littleforest]

Duplicate of #108